Fine mapping, introgression, and KASP marker development for powdery mildew resistance in watermelon using an interspecific RIL population (Citrullus mucosospermus × C. lanatus)

A major QTL conferring powdery mildew resistance was fine mapped to a 54.8 kb region on chromosome 2 using an interspecific RIL population (Citrullus mucosospermus × Citrullus lanatus). Four co-segregating KASP markers were developed and validated across multiple populations, demonstrating their utility for marker-assisted selection. Powdery mildew, caused by Podosphaera xanthii, is a major fungal disease that significantly affects watermelon production worldwide. Developing resistant cultivars through marker-assisted selection (MAS) offers an effective and sustainable strategy for disease management. In this study, a 54,772 bp quantitative trait locus (QTL) associated with powdery mildew resistance was mapped to chromosome 2 (30,111,475-30,166,247 bp) using an F₁₁ recombinant inbred line (RIL) population derived from an interspecific cross between the resistant C. mucosospermus line USVL531-MDR and the susceptible C. lanatus line USVL677-PMS. Genetic analysis revealed that resistance is controlled by a single dominant gene, supported by a 3:1 segregation ratio observed in F₂ populations. The mapped region harbored three lipoxygenase (LOX) genes and one 50S ribosomal protein L27-like gene. Four KASP markers were developed from SNPs located within four putative genes in the QTL region and were validated across multiple segregating populations, including the RIL (USVL531-MDR × USVL677-PMS) and two F₂ populations (USVL531-MDR × 'Sugar Baby' and PI 560003 × USVL677-PMS). These markers accurately differentiated resistant and susceptible individuals (R2 = 0.68-0.82) and exhibited 100% co-segregation with powdery mildew resistance in the RIL and two F₂ populations, demonstrating their utility for MAS. The identified QTL and validated KASP markers will facilitate MAS for powdery mildew resistance breeding and enable future gene cloning work.